1. Field of the Invention
The present invention relates to a transparent heat-shielding material and a method for making the same. More particularly, the present invention relates to a transparent heat-shielding film made of such a transparent heat-shielding material and featuring high transparency as well as excellent infrared-blocking properties.
2. Description of Related Art
In order to save energy and reduce carbon dioxide emissions, it is a common practice nowadays to carry out thermal insulation with a layer of heat-shielding material adhered to the glass panels of building and automobiles. A notable example of heat-shielding materials is metal oxides, whose physical properties contribute to effective thermal insulation and which have been widely used to block infrared radiation.
For instance, U.S. Pat. No. 5,385,751 discloses a fluorine-doped tungsten oxide as an infrared-blocking material. This material is made by chemical vapor deposition and is hence disadvantaged by a high manufacturing equipment cost and a high production cost.
Japanese Patent Application Laid-Open No. Hei 9-12338 discloses an infrared-blocking film which includes a composite tungsten oxide composed of the element tungsten and specific elements of, for example, the IVA group in the periodic table. This infrared-blocking film is made by a sputtering process in which the transparent glass substrate for supporting the film is exposed to high-temperature plasma. Because of that, the infrared-blocking film tends to be affected by high-energy ion beams in the plasma while being formed and may end up defective. To lower the defect density of the infrared-blocking film, an annealing apparatus is required for providing a heat treatment, which, however, adds to the complexity and cost of manufacture.
Japanese Patent Application Laid-Open No. 2003-121884 discloses a method for making a tungsten trioxide powder, wherein the method includes the steps of: dissolving tungsten hexafluoride in alcohol, separating a precipitate from the solution, and heating the precipitate at 100-500° C. to produce the tungsten trioxide powder. The tungsten trioxide powder thus obtained is applicable as an infrared-blocking material.
US Patent Application Publication No. 2006/0178254 discloses a method for making a composite tungsten oxide, wherein the method includes the steps of:
a) selecting one of a tungsten trioxide powder, a tungsten dioxide powder, a tungsten oxide hydrate powder, a tungsten hexachloride powder, and an ammonium tungstate powder;
b) selecting a powder of either an element M or a compound containing the element M, wherein the element M is more than one kind of element selected from the group consisting of H, He, alkali metals, alkaline earth metals, and rare earth elements;
c) mixing the powder of step a) with the powder of step b) at a specific ratio, adding alcohol or water into the mixture to produce a tungsten oxide hydrate, and drying the tungsten oxide hydrate;
d) performing the following two-stage heat treatment on the dried tungsten oxide hydrate of step c):
d-1) the first-stage heat treatment: heating the dried tungsten oxide hydrate at a sintering temperature of 100-850° C. in a reductive gas atmosphere, such as in an environment into which hydrogen gas (H2) is supplied; and
d-2) the second-stage heat treatment: once the dried tungsten oxide hydrate having received the first-stage heat treatment is cooled down to room temperature, heating it at a sintering temperature of 650-1200° C. in an inert gas atmosphere, such as in an environment into which argon gas (Ar) is supplied; and
e) grinding and thereby pulverizing the composite tungsten oxide which has completed the two-stage heat treatment, so as to produce composite tungsten oxide particles expressed by the general formula MxWyOz.
The composite tungsten oxide particles made by the method of the afore-cited US patent application publication can block infrared radiation sufficiently and are suitable for use as an infrared-blocking material. Nevertheless, the two-stage heat treatment required for making the composite tungsten oxide complicates the manufacturing process, which is undesirable.